1. Field of the Invention
This invention relates to an optical fiber transmission-line for transmitting multiple wavelength optical signals in a wavelength division multiplexing transmission system.
2. Related Background Arts
A wavelength division multiplexing (WDM) transmission system can effect high speed, high capacity optical communications by transmitting multiple wavelength optical signals. Because the transmission loss of a silica optical fiber used as an optical fiber transmission-line is small in the vicinity of the wavelength 1.55 xcexcm, and optical amplifiers for amplifying optical signals in the wavelength 1.55 xcexcm band have been put into practical use, multiple wavelength optical signals in the wavelength 1.55 xcexcm band are used in WDM transmission systems.
When in an optical fiber transmission-line for transmitting multiple wavelength optical signals there is chromatic dispersion in the optical signal band (the wavelength 1.55 xcexcm band), the pulse waveform of the optical signal spreads and transmission quality deteriorates. Therefore, from this point of view, it is desirable for the chromatic dispersion value in the optical signal wavelength band to be small. On the other hand, when the chromatic dispersion value in the optical signal wavelength band is substantially zero, the nonlinear optical phenomenon of four-wave mixing tends to occur, crosstalk and noise arise, and transmission quality deteriorates. The four-wave mixing generation can be suppressed by making repeater spans short and reducing optical signal power, but because this makes it necessary to provide many optical amplifiers the optical transmission system is expensive as a whole.
To deal with such problems, dispersion-managed optical fiber transmission-lines have been proposed, wherein, in one optical fiber, parts where the chromatic dispersion value at the wavelength 1550 nm is positive and parts where it is negative are provided alternately in the longitudinal direction. If this kind of optical fiber transmission-line is used, by making the average value of the chromatic dispersion value in the optical fiber transmission-line as a whole substantially zero, it is possible to suppress transmission quality deterioration caused by chromatic dispersion. And because at most points in the optical fiber transmission-line the absolute value of the chromatic dispersion value is not in the vicinity of zero, it is thought to be possible also to suppress transmission quality deterioration caused by four-wave mixing.
For example, in Reference 1, xe2x80x98Shiroki et al., xe2x80x9cEffects of Dispersion Fluctuation in WDM Systems using Dispersion Shifted Fiberxe2x80x9d, Proceeding of the 2000 Institute of Electronics, Information and Communication Engineers General Conference, B-10-210(1997)xe2x80x99, an optical fiber transmission-line wherein the chromatic dispersion value at the wavelength 1550 nm fluctuates in the longitudinal direction sinusoidally about the value zero is discussed. In Reference 1, for an optical fiber transmission-line dispersion-managed in such a manner, numerical analysis of the relationship between the period and the amplitude of the fluctuation of the chromatic dispersion value and transmission distances is carried out.
In Reference 2, xe2x80x98V. A. Bhagavatula, et al., xe2x80x9cNovel Fibers for Dispersion Managed High-Bit-Rate-Systemsxe2x80x9d, OECC ""98 Technical Digest, 15Cl-2 (1998)xe2x80x99, an optical fiber transmission-line wherein parts where the chromatic dispersion value at the wavelength 1550 nm is positive and parts where it is negative are provided alternately in the longitudinal direction with a period of 0.1 km to 6 km is discussed. In Reference 2 it is confirmed by experiment that by using an optical fiber transmission-line dispersion-managed in such a manner it is possible to suppress the four-wave mixing generation.
However, it has been discovered by the present inventors that, even when a dispersion-managed optical fiber transmission-line of the kind discussed in Reference 1 or Reference 2 is used, it does not always suppress the four-wave mixing generation effectively, and transmission loss sometimes increases.
It is therefore an object of the present invention to provide an optical fiber transmission-line with which the four-wave mixing generation can be suppressed effectively and also in which transmission loss is minimal.
To achieve this object, the invention provides an optical fiber transmission-line wherein, at the wavelength 1550 nm, in one repeater span there are at least four sign change positions at which the sign of the chromatic dispersion of the transmission-line changes; the absolute value of the average changing rate of chromatic dispersion in each sign change locality, a sign change locality being a range including a sign change position over which the absolute value of the chromatic dispersion is less than 2 ps/nm/km, is not less than 0.008 ps/nm/km/m and not greater than 4 ps/nm/km/m; the absolute value of the average value of the chromatic dispersion over the whole transmission-line is not greater than 2 ps/nm/km; the average value of the effective core area over the whole transmission-line is not less than 50 xcexcm2; the average value of the transmission loss over the whole transmission-line is not greater than 0.25 dB/km; and the average value of the polarization mode dispersion over the whole transmission-line is not greater than 0.2 ps/kmxc2xd.
This optical fiber transmission-line dispersion managed by having parts where the chromatic dispersion at the wavelength 1550 nm is positive and parts where it is negative provided in the longitudinal direction alternately. As a result of there being in one repeater span at least four sign change positions at which the sign of the chromatic dispersion at the wavelength 1550 nm changes, it is possible for the cumulative value of the chromatic dispersion to be made small. As a result of the minimum value of a preferred range of the absolute value of the average changing rate of chromatic dispersion in each sign change locality, a sign change locality being a range including a sign change position over which the absolute value of the chromatic dispersion is less than 2 ps/nm/km, being made not less than 0.008 ps/nm/km/m, it is possible to suppress the four-wave mixing generation. As a result of the maximum value of the preferred range of the absolute value of the average changing rate of chromatic dispersion in each sign change locality being made 4 ps/nm/km/m, it is possible to suppress an increase in transmission loss. Here, xe2x80x9cone repeater spanxe2x80x9d means a section between a transmitter (or a repeater) and a receiver (or a repeater) in which no repeater is provided.
By the absolute value of the average value of the chromatic dispersion over the whole transmission-line being made not greater than 2 ps/nm/km at the wavelength 1550 nm, transmission quality deterioration caused by waveform distortion is suppressed. By the average value of the effective core area over the whole transmission-line being made not less than 50 xcexcm2 at the wavelength 1550 nm, the nonlinear optical phenomena is suppressed. As a result of the average value of the transmission loss over the whole transmission-line being made not greater than 0.25 dB/km at the wavelength 1550 nm, the optical transmission system as a whole becomes inexpensive, because it is possible to make repeater spans long and reduce the number of optical amplifiers to be installed. And by the average value of the polarization mode dispersion over the whole transmission-line being made not greater than 0.2 ps/kmxc2xd at the wavelength 1550 nm, the transmission-line is optimized for carrying out high-speed optical communication. With an optical fiber transmission-line according to the invention, as a result of the several characteristics defined above being provided simultaneously, it is possible to suppress both waveform distortion caused by dispersion and the four-wave mixing generation; transmission loss is minimal; and the transmission-line can be used suitably even in an optical transmission system with a bit rate of over 40 Gb/s.
Preferably, in an optical fiber transmission-line according to the invention, the absolute value of the changing rate of chromatic dispersion in each sign change locality is not less than 0.04 ps/nm/km/m and not greater than 4 ps/nm/km/m. In this case, the four-wave mixing generation can be suppressed still further.
And preferably, in an optical fiber transmission-line according to the invention, the temperature dependency of the average value of the chromatic dispersion over the whole transmission-line is not greater than 0.005 ps/nm/km/K. In this case, even when, for example, the optical fiber transmission-line is to be installed on land and large temperature fluctuations of, for example, 50K are anticipated, the fluctuation of the average value of the chromatic dispersion over the whole of the optical fiber transmission-line is not more than 0.25 ps/nm/km, which is desirable in carrying out high-speed optical communication.
And preferably, in an optical fiber transmission-line according to the invention, the chromatic dispersion is varied by a core diameter and a fiber diameter being modulated in the longitudinal direction. Or, the chromatic dispersion is varied by a core diameter being modulated with a cladding diameter kept constant in the longitudinal direction. Both of these cases are suited to realizing an optical fiber transmission-line according to the invention.
The above and further objects and novel features of the invention will be more fully clarified from the following detailed description when the same is read in connection with the accompanying drawings. It is to be expressly understood, however, that the drawings are for the purpose of illustration only and are not intended as a definition of the limits of the invention.